The invention relates to a method of the noninvasive optical determination of the temperature of a medium, preferably a water-containing medium, where the medium to be analyzed is irradiated by infrared and/or visible light near an absorption line whose position depends on the temperature of the medium, and where the absorption of the light is measured near the absorption line and the temperature is determined from the measurement by comparison with calibration data. Medium in the context of the invention means in particular a water-containing medium, for example living tissue, and in particular (flowing) blood within the human body. Absorption in the context of the invention means, on the one hand, the absorption behavior measured, for example in transmission, but, on the other hand, also the backscatter behavior dependent on the absorption.
The determination of the temperature, for example of a human body, plays an important role in a variety of areas in medicine, for example during temperature monitoring of intensive-care patients. In practice, the noninvasive measurement of the body temperature by ear thermometers is frequently used, where this kind of measurement is limited to the “discrete” use, that is the measurement at regular intervals. To date, for a continuous temperature monitoring, invasive measuring methods are used in practice where probes or catheters are inserted or placed into the body.
Moreover, in connection with the noninvasive measurement of the concentration of blood components and in particular in connection with the measurement of the glucose concentration in flowing or pulsating blood, there is the need of temperature determination “in situ” because such measurements using calibration curves are usually dependent on the temperature (see DE 10 2006 036 920 and DE 103 11 408 [U.S. Pat. No. 7,251,518]). Here, different optical methods of the near infrared spectroscopy (NIRS) are known that, in a noninvasive manner by absorption changes of light in the infrared wavelength range, allow the measurement of the concentration of blood components and, for example, the measurement of the glucose concentration. The important fact is that the living tissue is substantially transparent in the red and infrared range for electromagnetic radiation so that it is possible to “look,” within this “biological window” into the tissue at depths of several millimeters up to several centimeters. With, for example, ultrasonic radiation, the target tissue can be localized in such a manner that optical absorption measurements in the localized tissue can be carried out to relatively great depth of the body (see DE 103 11 408 B3 and DE 10 2006 036 920).
Here, it should be noted that in the range of the so-called biological window, “discrete” water absorption bands are present that in the above described measurements of concentration of blood components are usually avoided. However, it is known that the position (and consequently the wavelength) of the absorption maxima and also the height of the absorption line (and consequently the extent and rate of absorption) depend on the temperature of the medium, for example water. For this reason, it has already been proposed to utilize the temperature dependency of the absorption in the range of the water absorption bands to determine the temperature of the water-containing medium. For this purpose it has been proposed to record the shifting of the absorption line spectroscopically (see K. H. Norris, Beltsville, Md. 20705, USA “Possible medical applications of NIR”). However, this known method is comparatively complicated because a complete spectrum always has to be recorded and consequently a “wavelength scan” is performed. Apart from that, the line shifting is relatively small so that very high spectrometer resolutions are required.
A similar method is known from US 2005/0083992 [U.S. Pat. No. 7,077,565]. There, the temperature dependency of the water absorption line is used for temperature determination at a wavelength of approximately 1450 nm. Also, in this known method, generally speaking, complete spectra are recorded over a comparatively large wavelength range, that is the complete measurement of the absorption line is carried out as well as the comparison with appropriate calibration data.